Leonardo Stern schrieb am Don, 22 Jun 2000:
> Hello L.A.M.E. people,
>
> 1 - Why ABR is much faster than VBR ?
because ABR (aka Safe-VBR) is some kind of constant rate encoding
with an unlimited (speek as much as the largest allowed bitrate
can give) bitreservoir.
> 2 - Anyone tested ABR x CBR quality ?
> 3 - I encoded a file with --abr 192 -js -h (lame 3.84 a1_2j from Dmitry)
> and the file have 183 kbps :
>
> 32 - 28 - 0.2%
> 64 - 1 - 0.0%
> 96 - 1 - 0.0%
> 112 - 9 - 0.1%
> | 128 - 250 - 2.2%
> |||||||||||||||| 160 - 3766 - 32.9%
> |||||||||||||||||||||||||||| 192 - 6490 - 56.7%
> || 224 - 564 - 4.9%
> | 256 - 267 - 2.3%
> 320 - 75 - 0.7%
> Average bitrate: 183.5
> Length: 04:59.13
> Total frames: 11451
>
> Why ?
you give the average bitrate lame tries to achieve,
btw it can be any integer between 4 and 310.
If for a frame lame couldn't use all available
bits, then lame will not waste them, lame will pack
the data in the smallest frame able to hold them.
In case that there is a hard to encode piece,
then lame will allocate more bits from the reservoir
and pack it into a larger frame.
So what is the difference between ABR and VBR?
With VBR lame does mostly 6 tries to find the smallest
amount of bits to achieve a given quality.
With ABR lame knows basically the number of bits, as
you defined them by calling lame. These bits together with the
perceptual entropy of each granule define the amount
of bits needed for a frame. Then lame does one try to
encode it as in CBR too. In CBR the frame sizes are fixed,
so the bitreservoir is limited. Now the frame sizes are
variable and lame can use as many bits from the bitreservoir
as needed (under the constrain that all has to fit into the
largest framesize ie 320 kbits).
Robert
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